1. Technical Field of the Invention
This invention relates to wireless communication devices, and more particularly to a mobile station that more efficiently measures signal strength of a set of channels in a wireless communications system.
2. Background of the Invention
FIG. 1 illustrates a typical wireless communications system 2. A plurality of mobile stations (MS) 10 are located in a geographical service area covered by cells C1 through C6. Radio base stations (RBS) 4 are positioned within the geographic area covered by the cells C1 through C6 and act as an interface between the mobile station 10 and the wireless communications system 2.
The radio base stations 4 are connected by a dedicated wireline to a mobile telephone switching office (MTSO) 6, also known as a mobile service switching center in certain wireless communications systems. The MTSO 6 is connected to a public switched telephone network (PSTN) 8 and may be connected to other MTSO's (illustrated by dotted line). The MTSO 6 controls the operation of the connected radio base stations 4 within the wireless communications system 2, such as setting up calls and coordinating the activities of the radio base stations 4. In addition, the MTSO 6 acts as switch to direct calls to and from the proper radio base station 4. The other MTSO's similarly control other radio base stations.
The wireless communications system of FIG. 1 has only a limited band of allowed frequency usage. To efficiently use this limited band, the geographical service area of the system is divided into a plurality of cells C1 through C6, with each cell assigned a set of channels in the allowed frequency band. Each set of channels is reused every k number of cells so that adjacent cells are assigned a set of disjoint channels to prevent interference. The set of channels for each cell includes both control channels and traffic channels. The traffic channels carry voice or data communication s and may be either analog or digital depending on the particular implementation of the wireless communication system. The control channels may also be either digital or analog and are used to provide information to and from the radio base stations 4 and the mobile stations 10 and control various functions of the mobile station 10. For example, the control channels are used to identify the particular cell in which the mobile is located, process a subscriber-originated call process, subscriber registration, and control other system processes.
As the mobile station moves from cell to cell, the mobile station 10 performs measurements of channels in neighboring cells while on a control channel to determine the best server. For digital control channels, the mobile station 10 measures bit error rate (BER) or word error rate (WER) or signal strength measurements in decibels. For analog control channels, measurements of signal strength in decibels are performed. If capable, the mobile station 10 may select either analog or digital control channels. In the control channel selection or reselection process, the radio base station 4 transmits a neighbor cell list of analog or digital control channels to the mobile station 10. The neighbor cell list includes information regarding control channels in neighboring cells for the mobile station to measure with regards to the control channel selection and reselection procedures.
If a call is in progress, new traffic channels are assigned to the mobile station 10 by the MTSO 6, called a handoff, without interruption of service. The mobile station 10 assists in the determination of traffic channel assignments by the MTSO 6 at handoff by measuring the signal strength of traffic channels in neighboring cells and reporting the data to the MTSO 6. This process is called mobile assisted hand-off (MAHO).
In a digital wireless communications system, such as systems described by EIA/TIA-IS-136 standard or IS-54-B standard, while on a traffic channel, a mobile station 10 is actively transmitting and receiving less than two-thirds of the time. During the idle time periods, a mobile station 10 is able to perform scanning and measuring operations in order to measure the quality of channels in neighboring cells. For digital traffic channels, signal strength measurements such as the bit error rate (BER) or word error rate (WER), while signal strength measurements in decibels are performed by the mobile station for analog and digital channels.
To initiate radio measurements by the mobile station 10 while on a traffic channel, the radio base station 4 transmits a measurement order to the mobile station 10. The measurement order includes a set of channels for the mobile station 10 to measure. During an idle period when the mobile station 10 is not otherwise transmitting or receiving traffic on the traffic channel, the mobile station 10 performs the measurements of the set of channels in the measurement order. The mobile station 10 generates a report of the performed measurements and transmits the report back to the radio base station 4. The MTSO 6 uses the report, among other things, to determine the channel assignments for the mobile station 10.
A conventional, prior art mobile station 10 is shown in FIG. 2. The mobile station 10 includes a controller 12 which controls the functions of the mobile station 10. The controller 12 typically includes a central processing unit (CPU) (not shown), memory (not shown), and I/O ports (not shown). The controller 12 processes voice or data signals to and from a transceiver 14. The transceiver 14 converts a voice or data signal from the controller 12 to a radio wave and also detects and demodulates a received radio wave into a voice or data signal. The transceiver 14 is connected to an antenna 16 for radio transmission and reception of radio waves.
The controller 12 is also connected to a microphone 18, speaker 20 and user interface 22. The microphone 18 includes a dynamic microphone, condenser microphone, or the like to transduce a user's voice into an electrical signal. An analog to digital converter (ADC) (not shown) convenes the electrical signal into a digital voice signal. The speaker 20 outputs a received voice signal, and typically includes a digital-to-analog converter (DAC) (not shown) and amplifier (not shown). The user interface 22 includes a display, such as an LED or LCD, and a keypad or other controls. A rechargeable battery 24 provides power to the mobile station 10.
A synthesizer 26 is connected to the controller 12 and transceiver 14. The synthesizer 26 generates signals at variable frequencies in response to an input frequency value from the controller 12. The generated signals are communicated to the transceiver 14 for reception or transmission of a channel about that frequency. The synthesizer 26 is includes a phase detector 28, loop filter 30, and voltage controlled oscillator (VCO) 32. A reference crystal 34 is connected to the synthesizer 26 and generates a reference signal 35 at a set frequency f.sub.r. The controller 12 controls the frequency output of the synthesizer 26 by transmitting a channel data signal 11 to the synthesizer 26 to store a divisor N value in a Divide-by-N Register 36. The controller 12 then transmits a control signal 13 to the synthesizer 26 to latch the divisor N value. In response, a frequency f.sub.o of output signal 33 from the VCO 32 is divided by N. The phase detector 28 compares the phases of the output signal 37 of the Divide-by N Register 36 and the reference signal 35 from reference crystal 34 to generate an error voltage (V.sub.e) signal 29 proportional to the phase difference between the two signals. The loop filter 30 is a low pass filter that filters the voltage error V.sub.e signal 29 prior to inputting the signal to the VCO 32. The frequency f.sub.o of the output signal 33 of the VCO 32 stabilizes to f.sub.o =N*f.sub.r and is transmitted to the transceiver 14.
The conventional operation of the mobile station 10 of FIG. 2 during a measurement operation of a set of channels is now discussed in reference to FIG. 3. In step 38, the mobile station 10 receives a measurement order or a neighbor cell list from the radio base station 4 over a traffic or control channel. The controller 12 waits until the next idle state to perform measurements of the channels designated, as shown in step 39. During the next idle period, the controller 12 determines the number of channels to measure, as shown in step 40. In step 42, the controller 12 stores data for the first channel to be measured in the Divide-by-N Register 36 of the synthesizer 26 and transmits a "latch new channel data" signal 13 to the synthesizer 26 in step 43. In response, the synthesizer 26 generates a VCO output signal 33 at a frequency f.sub.o specified by the channel data in the Divide-by-N Register 36. The VCO output signal 33 is communicated to the transceiver 14 which then receives a measurement command 15 from the controller 12 in step 44. The transceiver 14 measures the signal strength of the first channel and transmits a measured signal strength data signal 17 to the controller 12, as shown in step 46.
The controller 12 receives the measured signal strength data, as depicted in step 48, and decrements the number of channels to measure, as shown in step 50. The controller 12 determines if additional channels are to be measured in step 52. If no more channels are to be measured, the controller 12 again enters an idle period and is ready to transmit or receive on a traffic channel or an active state, as shown in step 54. Otherwise, the process returns to step 42, and the controller 12 loads the channel data for the next channel to be measured into the synthesizer 26. If there is not sufficient time to complete the measurements during the idle period between transmitting or receiving, the mobile station 10 must wait until the next idle period to continue the measurements.
This conventional process of measuring the signal strength of a set of channels requires much interaction by the controller 12. The controller 12 must individually store channel data for each channel to be measured in the synthesizer 26, transmit a latch data signal to the synthesizer 26, and initiate measurement by the transceiver 14 for each channel. As a result, this process requires valuable time and overhead by the controller 12.
The measuring operation in a mobile station needs to be performed quickly and efficiently, especially when a mobile station is measuring channels during an idle period, since there is limited time to perform the operation. A need has thus arisen in the industry for a more efficient method and apparatus to measure varying sets of channels by a mobile station.